WELDING THEORY - OD1650 - LESSON 1/TASK 1
The weldability of a metal is also affected by its thermal conductivity. In general, metals with
high thermal conductivity are difficult to weld because they transfer the heat away from the weld
so rapidly that the required temperature cannot be maintained at the joint.
Changes in temperature cause a metal to expand or contract and this also affects weldability.
When metals expand and contract at different rates, the internal stresses set up by these changes
can cause the joint to crack immediately, or to crack later under load.
Even when the weld joins identical metals, or metals having approximately the same coefficient
of expansion, the expansion and contraction may not be uniform throughout all parts of the
metal. These differences lead to internal stresses, distortion, and warping. Metal parts must be
free to move or a special weld sequence must be used. When heat is applied or withdrawn,
expansion and contraction set up high stresses, which may cause trouble in the weld itself or in
the adjacent base metal. In thin materials, uneven expansion and contraction may cause the
metal to warp. In heavy material, the stresses set up may exceed the ultimate strength of the
metal and cause cracking to occur in the weld, or in the metal next to the weld which is called the
heat-affected zone.
Even if the ultimate strength of the material is not exceeded by the stresses developed during
welding, the combination of welding stresses plus the stresses developed when the material is
placed in service may-be enough to cause failure of-the weld. It is for this reason that many
materials are stress-relieved after welding.
Another factor that influences weldability is the filler material used. The wrong electrode or an
incorrect welding process will make welding difficult or impossible, and it may lead to failure of
the part under service conditions. It is not always essential that the welding rod or electrode be
of the same chemical composition as the base metal; the important requirement is that the
combination of the filler metal and the base metal will make a satisfactory welded joint.
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